IEEE 802.11 Wireless LAN

This document introduces the IEEE standard 802.11 for Wireless Local
Area Network (WLAN). The document also makes a comparison to GSM cellular
network, where the cell size could be much larger. The paper discusses
the basis outlining the network planning process, such as frequency, scale
of mobility, transmission capacity needs and population variation. Finally
the total network planning process of a cellular network is introduced.
The question is how does the use of unlicensed frequency band affect the
WLAN network planning. Finally the study suggest what are the relevant
requirements for wireless communication with short range mobility. It also
suggests what GSM planning criteria can be omitted in the WLAN environment.

1 Introduction

The purpose of this document is to introduce the standard IEEE 802.11 for
Wireless Local Area Network (WLAN). Also the document compares the different
mobility scenarios of short range WLAN users with long range GSM users.
Also an important issue is the network planning process, due to the use
of unlicensed frequency band of WLAN.

In the first chapter the technologies, the cellular standard GSM and
WLAN are introduced. Then the aspects outlining the network planning process
- such as frequency, scale of mobility, transmission capacity needs, population
variation - are discussed. Finally the total network planning process of
a cellular network is introduced. An important question is how does the
use of unlicensed frequency band affect network planning in the WLAN environment.
Finally the study suggests what are the relevant requirements for wireless
communication with short range mobility. It also suggests what GSM planning
criteria can be omitted in the WLAN environment.

2 The Standards

2.1 Characteristics of
GSM and WLAN

GSM and IEEE 802.11 wireless LAN have very different characteristics. The
main differences between the two technologies can be found in extent of
coverage, services and therefore users and scale of mobility. The mobility
issues are discussed further in the section 3.2. These two wireless networks
can also be categorized as a cellular and a non-cellular network.

2.1.1 Extent of
Coverage

GSM is a cellular network typically offering a nation wide coverage depending
on the operator. With the possibility of international roaming GSM user
can use mobile services also globally. Also national roaming is supported
by GSM.

The GSM network is consisted of cells. The coverage area of each cell
is different in different environments. Macro cells can be regarded as
cells where the base station antenna is installed in a mast or a building
above the average roof top level. However, small cells or micro cells are
cells where the antenna height is under the average roof top level. Thus
the cell radius can vary depending on the antenna height, antenna gain
and propagation conditions from couple of hundred meters to several tens
of kilometers. Officially 35 km is the longest distance GSM specification
supports, though the specifications define an extended cell, where the
cell radius could be double [8]. Also indoor coverage is supported by GSM.
Indoor coverage can be built by using power splitters to deliver RF signal
from the antenna outdoors to separate indoor antenna distribution system.
When all the capacity of the cell is needed indoors, e.g. in shopping centers
or airports etc., the indoor coverage can be built by using antennas only
inside the building. In suburban areas the indoor coverage is usually origin
by the inbuilding penetration of radio signal, not by a separate indoor
antenna system.

WLAN is a standard offering a limited coverage for LAN users. Cell radius
is usually from few tens of meters to some hundred meters. IEEE 802.11
wireless LAN is a locally situated network - a local area network. The
coverage area is consisted of small islands and the purpose is certainly
not to offer a large coverage network like GSM. The coverage area is often
tailored according to the users own need and can also be temporary.

The specifications of wireless LAN outlines two possible modes of operations:
client/server and ad hoc mode WLAN. [4] In the client/server WLAN - also
often called as an infrastructure configuration - terminals communicate
with base stations or access points (AP), which form the coverage area.
The access points are further connected to the wired network. This document
concentrates mainly on the infrastructure WLAN when discussing network
planning issues. This is because a WLAN access point can comparable to
a GSM base station, but in the ad hoc mode the same station acts both as
an access point and as a station and needs therefore a different point
of view.

The coverage area of the client/server type WLAN network is usually
bordered upon a building or a campus and can therefore be comparable to
a single GSM inside cell. The main difference between the GSM and WLAN
technologies are, that to cover the whole building with the WLAN technology,
there has to be several access points depending on the building architecture,
wall materials etc. In the GSM solution the coverage area can be built
with a single base station by distributing the signal into antennas locating
in different rooms by using power splitters.

The specifications of WLAN defines also an ad hoc mode. In this mode
mobile terminals by themselves build the network. The coverage area is
built by the help of wireless adapters and is limited. [5, 13] In the ad
hoc mode the whole network is seen as movable, and it is independent of
any infrastructure unlike GSM or client/server type WLAN. It is also isolated,
because it has no interface to the wired network.

2.1.2 Services

GSM has been defined with the main purpose of voice services, operators
offer also data services at speeds of 9,6 and 14,4 kbit/s [8]. Although
a cellular network can never be regarded as completed, today the operators
can less and less compete with coverage area or quality of the network.
However, in these days the data services start to play a big role in operator
business. GSM service called High Speed Circuit Switch Data Service (HSCSD)
offers data services at speed up to 57,6 kbit/s, depending on the multislot
usage. Also this year some advanced operators will launch a new data service
named General Packet Radio Service (GPRS), which enables data communication
in the first phase at speed of 9,1-40,2 kbit/s, depending on the possible
multislot usage and the coding scheme used. Also short message service
and a set of value added services are specified in GSM system [8].

The services and therefore the users of WLAN are different from those
in GSM network. Due to the versatile set of GSM services – voice, data,
short message services etc. and the possibility of the large scale mobility
GSM can be regarded as a competitor to the wired public network. However,
WLAN supports only data communication and therefore can be seen as a potential
alternative or an extension to the wired LAN. [11]

The standardization process of IEEE 802.11 begun from the need to connect
wirelessly to the wired Ethernet based data communications network and
thus to offer mobility for the LAN users within a rather small area. One
of the main advantages of WLAN is that it provides LAN users an access
to real time information anywhere in their organization. Further one aim
was to reduce installation costs and thus achieve short and long term cost
savings. Installing a wireless system can be fast and the need to pull
cable through walls and ceilings can be eliminated. Wireless technology
allows also the network go where the wire cannot go.

The specification work of the standard IEEE 802.11 was started in 1990
and completed in 1997 by the Institute of Electrical and Electronic Engineering
(IEEE) [13]. The first phase of the standard IEEE 802.11 supports only
1 Mbit/s and 2 Mbit/s data rates. The first phase standard was followed
by an extension IEEE 802.11b, which supports data rates up to 11 Mbit/s
with the radio frequency technology direct sequence spread spectrum. [6,
12] However, the user does not have to purchase the radio operator's license
in order to use the frequency band 2,4-2,483 GHz, which is dedicated to
WLAN use, but it is also known as an ISM band (band for the industrial,
scientific and medical use) [14]. This means that anyone who have access
to a WLAN can buy and install an access point. This is a very problematic
basis from the network planning point of view.

2.1.3 Cellular
and Non Cellular -Networks

GSM and WLAN network can also be categorized into cellular and non-cellular
communication systems. [2] In a cellular system like GSM, the network is
consisted of cells. Each cell has it's own serving area and the user can
move over the cell boundaries by handovers. This can mean a very high infrastructure
cost, because several cells need to exist, before the user can move within
the network. Also very complex mobility protocols and signalling is needed
[9].

The non-cellular WLAN do not require an infrastructure when using ad
hoc mode of operation. Thus the complete system can be seen as movable.

2.2 GSM Physical Layer

The operation frequency of GSM lies in the 900 MHz band. The reserved bandwidth
can be divided into three areas [8]. Each of these frequency areas is located
in the 900 MHz band and uses separate frequencies for uplink and downlink
directions. However, there are also extra frequencies defined for the GSM
use in 1800 MHz band. The bandwidth specified for GSM use in 1800 MHz is
three times wider than the bandwidth of primary GSM in the 900 MHz frequency
band. The purpose of reserving frequencies also in the 1800 MHz band was
to fulfill the requirements of the increasing capacity need also in the
future [10]. Today e.g. in Finland there are several mobile operators which
operates either on both bands or in the 1800 MHz band only. The specifications
allow a handover between these two bands. Also there are several dual mode
cellular phones available in the market, which in the last resort enables
the mobility in the dual band network.

The physical layer of GSM defines the modulation scheme used, which
can be considered as a combination of frequency division multiple access
(FDMA) and time division multiple access (TDMA) [8]. The spectrum available
for GSM use is first according to FDMA technique divided into channels,
each channel 200 kHz of bandwidth. One or more frequencies are then assigned
to each base station (BTS). Every 200 kHz channel is further divided into
8 time slots. Each user is then assigned to a time slot or to a set of
time slots. Transmission is possible only during this time slot, after
that the user has to wait until he receives another time slot.

2.3 IEEE 802.11 Protocol
Layers

The specifications of IEEE 802.11 define two layers: layer one is called
Physical Layer (PHY) and layer two is called Media Access Control (MAC)
layer. Layer one specifies the modulation scheme used and signalling characteristics
for the transmission through the radio frequencies, whereas the layer two
defines a way accessing the physical layer [7]. The specifications of the
IEEE 802.11 layer two, called MAC layer, defines also services related
to the radio resource and the mobility management [13].

The standard defines three different physical layer characteristics
for WLAN: one infrared, and two RF transmission methods. This document
concentrates only on the two RF methods: direct sequence spread spectrum
(DSSS) and frequency hopping spread spectrum (FHSS). [12] However, due
to the operation in an unlicensed RF band, the spread spectrum modulation
must fulfill the requirements set by each country.

The chosen modulation techniques for the DSSS is Differential Bi and
Quadrature Phase Shift Keying (DBPSK and DQPSK). However, the FHSS uses
2-4 level Gaussian Frequency Shift Keying (GFSK) as the modulation schemes.
Depending on the modulation scheme used both direct sequence and frequency
hopping spread spectrum supports data rates of 1 Mbit/s and 2 Mbit/s. As
mentioned in chapter 2.1.2 also the data rates of 8 Mbit/s and 11 Mbit/s
with the DSSS are possible to achieve while using WLAN that supports the
standard IEEE 802.11b [6, 12]. The operation frequency of both RF methods
is 2,4 GHz. [5, 13]

In order to wireless LAN devices to be interoperable, they have to have
the same physical layer standard. Therefore a DSSS equipment is not capable
of communicating with a FHSS based equipment.

3
Basis for the Network Planning Process

The section four describes the major points outlining the network planning
process, both from the GSM and the WLAN technology point of view. The WLAN
part has been written from the infrastructure WLAN point of view unless
otherwise stated. However, some problems concerning planning an ad hoc
WLAN has been discussed in the section 4.6.2.

3.1 Frequency Range

The frequency used in wireless communication is strongly affecting the
range of coverage. In different frequency band there are different circumstances
to the RF signal to propagate. However, the used spectrum makes anyway
the environment of GSM and WLAN different, therefore propagation issues
of different frequency band is not compared in this document. This section
states the allocated frequencies for WLAN use, as well as gives an example
of a frequency plan of a DSSS WLAN.

As mentioned earlier the specified frequencies for GSM use lie in the
900 and 1800 MHz. However, both WLAN RF methods are planned to operate
in the unlicensed 2,4 GHz frequency band occupying typically a bandwidth
of 83 MHz between 2,4-2,483 GHz. The maximum allowed antenna gain is 6
dBi. Also different frequencies are approved in different countries. [4,
12]

The direct sequence spread spectrum generates a redundant bit pattern,
length of eleven bits, which then multiplies each data bit to be transmitted.
The bit pattern is also often referred to a chipping code. Only one chipping
code is used [12] unlike in CDMA system, which has a set of codes. The
receiver must be tuned into the right frequency in order to be able to
receive the information being broadcast. Direct sequence spread spectrum
is at the moment the one that most wireless spread spectrum WLANs uses
[14].

In Europe there are 13 frequencies for DSSS use, the center frequencies
and corresponding channel numbers are listed in the table 3.1. The channels
are overlapping and the receiver adjacent channel rejection demand is 35
dB, which needs 30 MHz gap between the neighbouring channels. [4] The center
frequencies are located in 5 MHz interval and according to the adjacent
channel rejection requirement, there has to be five channels between, in
order to avoid interference caused by neighbouring access points. This
frequency reuse plan means also that an access point having six neighbours,
alternate neighbour uses the same frequency as illustrated in figure 3.1.
This frequency design demand has to be taken into account, when choosing
operating frequency to the DSSS equipment. The problem is that there is
no controlling organisation that follows the reuse of frequencies, like
regulator and operators in the case of GSM.

Table 3.1. DSSS frequency plan used in Europe.[4]

Channel number

Frequency

1

2412 MHz

2

2417 MHz

3

2422 MHz

4

2427 MHz

5

2432 MHz

6

2437 MHz

7

2442 MHz

8

2447 MHz

9

2452 MHz

10

2457 MHz

11

2462 MHz

12

2467 MHz

13

2472 MHz

Figure 3.1. An example of a frequency plan of an infrastructure
network using DSSS.

In the frequency hopping spread spectrum the carrier changes the frequency
used according to a pattern known both by the transmitter and the receiver.
There are 79 non-overlapping channels specified, each channel occupying
1 MHz. 26 different pseudorandom hopping pattern are specified. [4] The
hopping rate is specified by the regulator of each country. Therefore WLAN
equipment must meet the requirements of the country where it is sold. [12]
The user can choose the frequency hopping pattern used by a FHSS equipment.

The frequency hopping spread spectrum is a technique that enables coexistence
of multiple networks or other devices in the same area. This is due to
the resistant to multipath fading through the inherent frequency diversity
system. Also the frequency hopping design criteria described in [1] have
the desirable effect on the multipath fading resistant.

Certain differences can be found between the direct sequence and the
frequency hopping spread spectrum. DSSS supports bigger data rates (up
to 11 Mbit/s) and FHSS equipment are usually inexpensive compared to DSSS
equipment. [6, 12, 14]. In a case where high throughput is needed and the
interference is not a problem DSSS is the better choice.

The defined frequency band or the frequency channels available sets
the practical limit for the capacity of a GSM network. However, this is
rarely the case, typically the defined bandwidth is divided between numerous
operators by the regulator of each country. Thus the regulator defines
the maximum capacity of the network.

As stated the use of the ISM band requires no operator license. The
technology choice of spread spectrum is different from the one used in
GSM. Therefore the actual capacity limit is not defined according the available
bandwidth but the interference level. Further packet mode traffic, which
is used in WLAN, does not know the specification of congestion. Therefore
the available transmission capacity depends strongly on the interference
level. The problem of a WLAN is that there could be interference not only
caused by the other access points located in the own network, but also
caused by neighbouring WLAN or other equipment using the same frequency
band. There is not much to do with the interference level in the latter
case.

3.2 Mobility Aspects

In this chapter the mobility aspects – scale of mobility and speed of a
mobile – of GSM and WLAN are discussed.

The main difference is that GSM network is consisted of cells and therefore
requires a handover mechanism. However, the specifications of WLAN do not
define a handover mechanism [13], therefore the roaming is transparent
to the MAC layer of IEEE 802.11. This means that in case of an Infrastructure
WLAN from the wired network point of view there seems to be two node's
having different IP addresses, but the IP addresses share the same domain
network name. In case of ac hoc WLAN the concepts of handover or adjacent
access point can not be found, because the stations act both as an access
point and as a station. As pointed out the whole system can then be seen
as movable.

3.2.1 Scale of Mobility

GSM is a network consisted of cells. The purpose of the operator usually
is to offer a nation wide coverage and with international roaming the user
can use mobile services also globally. The mission for several operators
and vendors is that the user can use his phone where ever and whenever.
The need to move within the network is essential otherwise the concept
of mobile services would not exist.

In order to ensure the large scale of mobility, several functions has
had to be designed. The mobility management of GSM is defined in the layer
three specifications, and it is very complicated compared to that of a
WLAN user. As stated the smallest unit of the radio network is a cell.
Several cells build up a location area (LA). The concept of LA has been
a necessity to define, in order to handle large scale mobility of the user.
If a powered mobile station (MS) is having an incoming call, it is paged
through the pacing (PAGCH) channel of a cell. It would be a waste of bandwidth
if the paging has to be done in every incoming call through every cell
of the network. Thus the network knows situation of each user in an accuracy
of a location area. [9] The operator decides the size of the location areas
in order to balance the signalling traffic of the network.

Several cells are connected to Base Station Controllers (BSC) and further
BSCs are connected to Mobile Switching Centers (MSC). The number of BSC
and MSC is a matter of the vendor but also a matter of optimisation and
maximizing the reliability of the network and therefore not discussed here.
When the MS moves from cell to cell, and from a location area to another
area, additionally two subsequent registers – HLR (home location register)
and VLR (visitor location register) – are required.

Radio Resource Management, which controls the call setup, maintenance
and termination, as well as it also handles the signalling used in handovers.
Different types of handovers are supported. Intra cell handover, which
can occur within a cell or within a frequency from time slot to another
time slot. This is normally done in case of interference. The other type
is a handover between different cells. The cells can be situated in different
BSCs and/or different MSC. This of course affects the signalling need in
handover. This type of a handover is the way to move within the network.
Despite of the ability to move within the network by handovers, handovers
can be used to control the traffic load of the cells.

The MS measures the RX level of the surrounding BTSs according to the
neighbouring list and reports the measurements to the BSC, where the handover
decision is done. The suitable choices of handover and power control parameters
makes the BSC to keep the connection alive by the help of power control.
The handover attempt is not done until the user really is in the border
of the cell.

---

On the other hand the mobility concept of WLAN is slightly simplier.
Typical users of WLAN infrastructure mode can be e.g. doctors seeing his
patient, the patient information can be written straight to the patient
register in the laptop by the help of WLAN. Also students at the universities
or people participating conferences could use their laptops as a notebook
by the help WLAN. Ac hoc WLAN offers no access to the wired network, instead
of that the stations can share files, which could be useful e.g. in confrerences.
These are indoor solutions, but the specifications tell no reason why WLAN
coverage can not be built outdoors as well. Anyway in cases like this it
could be supposed that the WLAN users do not need to move within the network
in that extent what the user is expected to move in GSM network. This is
perhaps because WLAN does not support speech transmission and data service
users are not expected to move in a large scale. Thus it could be assumed
that a typical WLAN user would be a user that moves from location to location,
but uses the WLAN equipment only at a fixed location. Therefore no complex
mobility scenario is needed. Next the basic services supporting mobility
are shortly described.

Association is a basic service that enables the connection between the
station (STA) and the access point (AP) in an infrastructure WLAN. An access
point is basically a radio base station that covers an area of about 30…300
m depending on the environment. An access point and its associated clients
form all together a Basic Service Set (BSS). [13] Though no handover mechanism
is specified in the standard, the standard introduces a service called
reassociation, which is related to the roaming from one BSS to another.
Two adjoining BSS form together an Extended Service Set (ESS) if they share
the same ESS identity (ESSID). This is the case when roaming is possible.
[4] Thus the parameter ESSID can be referred to the concept of neighbouring
cell in GSM network.

An Independent Basic Service Set (ad hoc mode, IBSS) is the most basic
type of IEEE 802.11 WLAN. At the minimum it is consisted of two stations.
The network is often formed without pre-planning is alive until either
station is moved away from the others coverage area. [4]

3.2.2 Speed of
the Mobile

According to the mobility theorem presented in [3] the speed of the mobile
is a crucial fact affecting the time the mobile station stays in the coverage
area of a cell. If the user reaches the border of an individual cell by
the velocity v, the velocity v' is the time the user reachers
the border of N cells. In the [3] it is certified that the ratio
of v/v' follows the square root of N.

It is assumed the N to be 100. The velocity v' can now
be calculated to be one tenth of the velocity v. Thus the speed
of the mobile is more crucial issue to the user of a wireless local area
network than to a mobile phone moving within a network covering larger
entities. Thus the aim of the WLAN coverage planning is often to maximise
the coverage area of an individual WLAN access point.

3.3 Transmission
Capacity Needs

A good question is how much transmission capacity does a single application
need? Of course this depends very much on the application. In case of GSM
voice service, we know exactly what the voice transmission over the air
needs. [8] Further if we know, how often the user does take a call, we
can calculate from Erlangs formula the needed capacity.

In case of data services the estimated need of capacity is not as straight
forward. It is not possible to measure the amount of capacity needed from
the used application. If we know that the www-application would demand
say 40 kbit/s from the GPRS service, this does not take into account how
unexpectedly the user can behave. He can suddenly found some interesting
link transmitting e.g. video picture and thus the demand for transmission
capacity still increases. However, this kind of a phenomena does not take
into account that the behave of one user affects also to the behaves of
the other users. Let us think about the www-surfer, that has found an interesting
video picture. As his transmission capacity need increases, the transmission
capacity of other www-surfers in the same cell decreases. So the behaving
scheme of a data user can be regarded as a very complex mathematical model
and thus this theme is suggested to a subject of further study.

It can also be stated the packet data user - e.g. GPRS or WLAN user
- does not experience the congestion in the way that the circuit switched
user (e.g. GSM voice) does. The congestion of a packet data transmission
is experienced as the transmission rate of the service becomes slower.
So practically there is always room for another user using packet service.
Only the speed of the service can not be quaranteed.

3.4 Power Control

The power consumption of the mobile station is a crucial issue when using
wireless services. Both standards GSM and WLAN have taken this point into
account. The closer the user is to the base station or access point the
smaller transmission power is needed. The aim of the power control is simply
to minimise the power used, but still to keep the quality of the connection
good enough [10].

The GSM specifications specifies both downlink and uplink power control.
The downlink power control mainly decreases the interference level of the
network and thus improves the quality of the network. BTS power control
is not obligatory and thus it is up to the decision of the operator. On
the other hand the uplink power control improves the battery lifetime of
a mobile phone. The GSM power control is controlled by BSC and it is based
on the measurements done by the BTS and MS. The measurements are done in
480 ms interval. If the average result of the measurement exceeds the threshold,
the power change is done in 2 dB steps, each step takes 60 ms. The power
control range of a GSM mobile phone in the 900 MHz band is from the 13
dBm (20 mW) to the maximum specified 39 dBm (8 W). [8]

WLAN is a symmetrical system, which can be noticed from the stations
ability to act as an access point in the ad hoc mode. Thus the power control
functions similarly from the stations and the access points of view. In
the DSSS WLAN the maximum transmission power is specified to be 100 mW
(EIRP), whereas the minimum power level shall be no less than 1 mW. Four
different power levels are defined. For example in an infrastructure network
the station selects it's power management mode by a parameter, by which
the station informs the access point through a successful frame exchange.
In a case when there is nothing to be received or transmitted the station
is able to fall into a sleep mode and then consumes very low power. In
this mode the station is not able to transmit or receive any data. However,
in a low power mode the station listens periodically beacons sent by the
access point. In this way the access point is announcing the stations of
it's existence. [4]

In the FHSS WLAN the power level are specified as follows. The EIRP
of the maximum transmission power is 100 mW and the smallest transmission
level is 10 mW. [4] The specified power levels are regulated by local regulators
and are different from continent to continent. The values presented here
are the values used in Europe.

It could be assumed that the speed of the mobile also affects the users
ability to recover from the fading point. Thus the mobile station moving
at high speed recovers from the fading caused by the multipath propagation
more quickly than the slow moving mobile. Thus the power control can regarded
more critical to the slow moving user.

3.5 Population
Variation

One of the corner stones of the network planning is to define the capacity
of the network according to the peak traffic. However, there are times
when capacity needed is remarkable bigger. Exhibitions, rock concerts,
sport happenings, even a shopping day before christmas could cause a remarkable
variation in offered traffic of the network. The GSM operators usually
takes the biggest happenings into account by building extra temporary capacity
to the network. This can be done e.g. using movable base stations or simply
adding the extra capacity to the specific cells covering the happening
area.

In case of WLAN user or packet data user, the capacity planning can
be omitted, because there is always room for another packet data user,
only the offered transmission rate decreases when the amount of users increases.
So the document suggests, that the population variation does not have to
take into account when planning a WLAN network. On the other hand in a
case where are several hundreds of concurrent users and a certain level
of data rate must be quaranteed, the use of several access point instead
of one should be considered, although sufficient coverage area could be
created with one access point.

3.6 Timing Advance

The delay of radio propagation time from transmitter to receiver is limiting
the distance of GSM connection. Therefore the MS transmission point
of time has be changed depending on the distance between the MS and the
BTS. This is done by the parameter timing advance. The MS transmission
point of time has to adjusted, in order the MS transmission to hit the
right time window assumed by BTS. The range of the value is between 0...63.
One unit corresponds to 0,55 km. Thus the maximum distance from the transmitter
is limited to 35 km. [8]

The resolution of the parameter timing advance (0,55 km) is bigger than
the specified maximum range of Wireless LAN. Therefore no timing advance
parameter is needed in short range communication system like WLAN.

4 Network Planning
Process

This section describes the corner stones for the network planning process
of a cellular network, which is typically a land wide network, planned
for both outdoor and indoor use. It is further discussed, if a planning
process for a WLAN network is needed at all, and if needed, what GSM network
design principles can be omitted in the WLAN environment.

4.1 Targets for the
Network Planning

The targets of the network planning are depending on the offered services,
the user needs and therefore expected traffic load. Also the frequency
band available and the chosen infrastructure outlines the structure of
the network. The main targets of the network planning is to quarantee service
with a quality good enough and to offer capacity with a sufficient low
congestion. The good quality could mean a low dropped call percentage,
low quality figures (quality level can be measured between 0...5, 0 being
the best class), but also it could mean a good indoor coverage. The demand
for coverage depends strongly on what the customers are used to. If the
customers expects as good as coverage as the fixed network offers indoors,
the mobile station should be able to take a call also everywhere indoors.

Network planning is a ongoing process, which has to be optimised economically.
However, one target is also to take into account the possible capacity
need in the future right from the beginning. The different steps of the
planning process are illustrated in the figure 4.1.

Figure 4.1. The network planning is a ongoing process,
which is consisted of various steps and affected by various parameters.

4.2 Coverage Planning

The coverage of a cell means an area, where the user can make a call without
problems. The coverage planning is based on the wanted coverage level and
its demanded amount of base stations. In the coverage planning the topography
of the serving area is good to take into account. Also indoor coverage
target in the cities affects the amount of needed base stations. As a result
of the coverage planning will be the predicted coverage area and the transmission
powers of the cells. Different propagation models and planning software
are needed in order to predict the radiowave propagation and therefore
coverage.

4.3 Capacity Planning

The coverage and capacity planning go hand in hand in GSM network planning
process. Seldom, hardly ever the network is planned and built to be ready
right in the beginning. Usually the operator plans and builds the network
in an incremental way. This means that the operator may start to offer
service with not a very dense network, perhaps offering no indoor coverage
at all. Usually first the network is consisted of big cells, in order to
offer as wide as coverage as possible. The capacity is seldom a problem
at first. As the amount of customers increases also the need for better
coverage area and capacity increases. Then usually bigger cells will be
divided into several small and micro cells in places where high capacity
is needed. This leads to a network, where micro and small cells are used
in the cities and big cells or smaller cities in the countryside. Therefore
the goal of GSM coverage planning is not necessary to maximise the coverage
of an individual cell. The denser the network, the bigger the offered capacity.

In the capacity planning two different schemes can be distinguished:
capacity planning of a certain area entity and capacity planning of an
individual cell. The capacity of a GSM cell is consisted of TRX's. One
TRX contains 8 channels or timeslots, and TRX can carry 4 Erlangs traffic
with 2 % time congestion. Thus accepted congestion level affects the capacity
planning. Some level of congestion can be acceptable e.g. in the city areas
where cell overlapping can be found in a great extent. However, the offered
capacity is not only a matter of excisting channels, the operator can affect
the amount of needed channels by the help of various parameters by which
the operator can share the offered load between neigbouring cells.

4.4 Frequency Planning

The capacity plan gives as a result the number of TRXs, whereas the frequency
plan gives the frequency in used in every TRX. Frequency planning step
is also affected by coverage plan. The basis to plan the frequencies are
the same and neighbour channel rejections, which are defined in the specification.
Also the frequencies allocated to the operators use are essential to know.
In GSM network the frequencies are repeated in a sufficient distance. The
distance depends on the specified channel rejection levels and it is also
further affected by the topography and obstacles of the environment.

4.5 Parameter
Planning and the Follow Up

In addition to the steps of coverage, capacity and frequency planning,
there are still a lot to be optimised. Once the planned BTSs functions
in the network, the important phase to the planner is to investigate how
successful was the plan. This can be done e.g. by making test drives around
the measured area with a test mobile phone and/or following cell specific
measurements reported by the network. Then the possible changes needed
in antenna configuration, capacity or frequency plan is now to be done.
Also the following up as well as the parameter planning of the network
is important. By the parameters the operator can affect e.g. the handovers,
BTS and MS power control.

4.6 Relevant
Requirements for IEEE 802.11

4.6.1 Infrastructure
WLAN

The question is that does the WLAN network need a multi phased planning
process like used in GSM cellular system. The costs of an access point
and a wireless adapter needed in the e.g. laptop connected wirelessly are
probably lower than one cellular planners monthly salary costs. Further
we can ask who would be the planner, if a user can buy a WLAN access point
and install one by himself. Also there is no organisator who would supervise
the frequency reuse as the frequencies lie in the unlicenced frequency
band.

Now in case of infrasructure WLAN the situation is that the user who
have access to the LAN and wants to make that wirelessly, buys an access
point and installes it. The user makes a coverage plan by simply installing
the equipment e.g. to the wall and if the place is not optimised he probably
tries another place. Further the user makes a frequency plan by selecting
randomly the channel, if there are lot of interference he will probably
change it. So the coverage planning and frequency planning steps can be
found in WLAN planning "process". However, the planning step of capacity
can be omitted in the WLAN environment. As stated previously this is from
the reason, that there is always room for another WLAN packet user. The
information is transmitted in packet mode, therefore the user does not
experience any congestion, only the transmission rate varies. However,
as stated previously if a certain transmission data rate is to be quaranteed
and there are e.g. several hundreds of concurrent users there might be
a need for using several access points instead of one, although sufficient
coverage area could be created with one access point. Overall it can be
stated that a distinctive capacity design principle can be found from the
GSM world but cannot be found from the WLAN environment.

The problem of the WLAN planning is also that the user is not necessarily
familiar with technical equipment. In a case the user installes an access
point and tries to make a wireless connection to the LAN, but does not
get it, he probably does not know what the problem is. Is the used frequency
too interfered or perhaps the installation place for the access point is
not good enough? Further what is making the interference? However, the
writer does not have experience of testing a live connection in WLAN, but
assumes that the biggest problem at the moment is the interference caused
by other equipment (not WLAN) using same frequency band. Micro owens, Blue
Tooth equipment or different electronical equipment using 2,4 GHz (e.g.
door opening equipment) could cause interference.

Relevant requirement for the IEEE 802.11 infrastucture network planning
would be that the intelligence of the frequency planning should be in the
equipment itself. In this way the level of interference would be minimised.
The other demand could be that there would be a larger frequency band dedicated
to the WLAN use than available today, so the interference would be spread
into a larger area and thus the average interference level would be lower.
Both demands suggested in the document are taken into account in the next
generation WLAN called HiperLAN2, which uses higher frequency (5 GHz) and
supports also interfunction with third generation cellular standard UMTS.
In HiperLAN2 the access point automatically selects an appropriate channel
and dynamically changes it according to the level of interference [6].

4.6.2 Ad Hoc WLAN

In the ad hoc mode several wireless stations (e.g. laptops) can get together
in a local area (e.g. conference room) and form their own wireless network.
Through an ad hoc mode, the users can share documents and devices.

Generally the problems an ad hoc WLAN is facing could be the same as
in the infrastructure mode. The used frequency is perhaps interferred or
the coverage area is not large enough etc. However, the writer assumes
that the coverage is not a usual problem in an ad hoc mode. This is because
it is assumed that the users have decided to meet each other in some conference
room and the only demand for the coverage is that the stations can hear
each other in that specific room. So the network is typically created spontaneously
and is alive until a station has moved out of the coverage area of other
stations.

The question of the network planning in an ad hoc mode is irrelevant.
No network planning process of an ad hoc WLAN can be addressed. So rather
than discussing the problem of network planning of an ad hoc WLAN, the
more suitable question is that how the stations configure themselves to
act both as an access point and as a station. The ad hoc WLAN is using
a certain frequency and thus a station that wants to be a member of the
network should use the same frequency. So can an accidental WLAN station
be a member of any ad hoc WLAN it has founded? Further, if the used frequency
is too interferred and the connection between stations is poor, an interesting
question is that what happens if one of the stations suddenly decides to
the try another frequency? Is this somehow possible or are the frequency
selection and the possibly frequency change a common decision of all the
members of ad hocWLAN ? These are questions that are left under further
study.

5 Conclusion

GSM is a cellular network standard. The standard defines a network usually
covering large entities, enabling large scale of mobility and offering
a versatile set of services - e.g. voice and data - and thus competing
with the wired public telephone network. As the user demand for higher
data rate increases, new data services has been defined. The maximum range
of GSM communication is 35 km.

The standard IEEE 802.11 WLAN defines to the LAN user (e.g. laptop)
a mean to connect wirelessly the Ethernet based LAN. Also wireless connection
(ad hoc mode) between stations are defined. The main advantages of WLAN
is to offer mobility for the LAN users though within a rather small area.
Cell radius is usually from few tens of meters to some hundred meters.
WLAN provides LAN users an access to real time information anywhere in
their organization. Further one aim was to reduce installation costs and
thus achieve short- and long-term cost savings. Installing a wireless system
can be fast and the need to pull cable through walls and ceilings can be
eliminated. Wireless technology allows also the network go where the wire
cannot go.

The document has compared various parameters - such as frequency range,
mobility aspects, transmission capacity needs, power control, population
variation and timing advance - outlining the network planning of
GSM and WLAN. The coverage area of a WLAN is usually very limited, usually
bordered upon a building or a campus and can therefore be comparable to
a single GSM inside cell. Because of the limited coverage the speed of
the mobile is stated to be crucial to WLAN user, as well as the power control.
The documents handles the transmission capacity need of a user using data
communication. It is stated that the behaving of an individual data user
affects the behaving of the other data users sharing the resources of the
same cell. The conclusion is that it is difficult to estimate the need
for transmission capacity from the used application. Because of WLAN data
communication uses packet mode communication instead of GSM's circuit switch
mode, the WLAN user does not experience congestion in the same way as the
circuit switch user. Only the offered transmission rate varies. This is
the reason why there is no need to take the population variation of WLAN
into account. Further it was stated that no timing advance parameter is
needed in short range wireless communication like WLAN.

The targets of the GSM network planning are depending on the offered
services, the user needs and therefore expected traffic load. Also the
frequency band available and the chosen infrastucture outlines the structure
of the network. The main targets of the network planning is to quarantee
service with a quality good enough and to offer capacity with a sufficient
low congestion. The GSM network planning process is consisted of three
phases: coverage, capacity and frequency planning. The phases are strongly
dependent on and affected by each other.

An infrastructure WLAN does not need a multi phased network planning
process like used in GSM cellular system. This can be argued with the costs
of the network planning. Also there is not going to be an organisator who
would supervise the frequency reuse as the frequencies lie in the unlicenced
frequency band. The steps of coverage planning and frequency planning can
be found in WLAN planning "process". However, the planning step of capacity
can be omitted in the WLAN environment for the same reason which was mentioned
when defined the rules for the population varation. It was also stated
that the question of the network planning process in an ad hoc WLAN is
irrelevant.

Relevant requirement for the IEEE 802.11 infrastucture WLAN planning
would be that the intelligence of the frequency planning should be in the
equipment itself. Also the more channels are available for WLAN use the
lower the interference level would be. Both demands suggested in the document
are taken into account in the next generation WLAN called HiperLAN2, it
also supports interfunction with third generation cellular standard UMTS.